Patent Document 1 listed below discloses an X-ray imaging device (X-ray image forming apparatus) which images an X-ray image of teeth, etc., of an examinee by using an image sensor including a CCD (Charge Coupled Devices). This image sensor includes, in addition to the CCD for imaging an X-ray image, a monitoring photodiode (X-ray detector) for detecting an X-ray irradiation timing. Based on an output signal from the X-ray detector, a trigger signal showing the start or the end of X-ray imaging is generated. In detail, when the value of the output signal exceeds a predetermined threshold, a trigger signal showing the start of X-ray imaging is generated.    Patent Document 1: Japanese Published Examined Patent Application No. 3335350